UNIT-2

THE REFERENCES MODEL FOR

NETWORK COMMUNICATION

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SYLLABUS

2.1 OSI model & function of each Layer

2.2 TCP/ IP model

2.3 Connection oriented v/s Connectionless

approach

2.4 Comparison of OSI & TCP/IP Models

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2.1 OSI MODEL & FUNCTION OF EACH LAYER

Open Systems Interconnection Basic Reference Model

(OSI Reference Model or OSI Model) is an abstract

description for layered communications and computer

network protocol design.

It was developed as part of the Open Systems

Interconnection (OSI) initiative.

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CONTINUE…

In its most basic form, it divides network

architecture into seven layers which, from top to

bottom, are the Application, Presentation,

Session, Transport, Network, Data-Link, and

Physical Layers.

It is therefore often referred to as the OSI Seven

Layer Model.

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APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

Application to Application

Application to Application

Application to Application

Process to Process

Hop to Hop Switch

Hop to Hop

Physical Medium

Hub and Repeater

Router Source to Destination

Source to Destination

OSI MODEL'S 7 LAYERS

HOST AND MEDIA LAYER

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DATA, PROTOCOL & ACTIVITIES

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TCP/IP Suit OSI Layers

Application Telnet, FTP, SMTP, HTTP, DNS, SNMP, Specific address etc…

Session

Presentation

Transport SCTP, TCP, UDP, Sockets and Ports address

Network IP, ARP/RARP, ICMP, IGMP, Logical address

Data Link IEEE 802 Standards, TR, FDDI, PPP, Physical address

Application

Session

Presentation

Transport

Network

Data Link

Activities

To allow access to network resources

To establish, manage, and terminate

session

To Translate, encrypt, and compress

data

To Provide reliable process-to-process

Message delivery and error recovery

To move packets from source to

destination; to provide internetworking

To organize bits into frames; to provide

Hop-to-hop delivery

Physical Medium, Coax, Fiber, 10base, Wireless

Physical To Transmit bits over a medium; to provide

Mechanical and electrical specifications

PHYSICAL LAYER

One of the major function of the physical layer is to move data in the form of electromagnetic signals across a transmission medium.

Its responsible for movements of individual bits from one hop (Node) to next.

Both data and the signals can be either analog or digital.

Transmission media work by conducting energy along a physical path which can be wired or wireless 8

10101000000010111 110

From data link layer

10101000000010111 110

To data link layer

Physical layer Physical layer

Transmission medium

CONTINUE…

Concerned:

Physical characteristics of interface and medium (Transmission medium)

Representation of bits (stream of bits (0s or 1s) with no interpretation and encoded into signals)

Data rate (duration of a bit, which is how long it last)

Synchronization of bits (sender and receivers clock must be synchronized)

Line configuration (Point-to-Point, Point-to-Multipoint)

Physical topology

Transmission mode (Simplex, half duplex, full duplex)

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DATA LINK LAYER (HOST TO HOST)

Data link layer is responsible for moving frames from one hop (Node) to the next.

Concerned:

Framing (stream of bits into manageable data units)

Physical addressing (MAC Address)

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Data H2 T2

From network layer

Data H2 T2

To network layer

To physical layer From physical layer

Data link layer Data link layer

Flow Control (mechanism for overwhelming the receiver)

Error Control (trailer, retransmission)

Access Control (defining master device in the same link)

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NETWORK LAYER (SOURCE TO

DESTINATION)

The network layer is responsible for the delivery of individual packets from the source host to the destination host.

Concerned:

Logical addressing (IP Address)

Routing (Source to destination transmission between networks)

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Data H3 Packet

From transport layer

To data link layer

Data H3 Packet

To transport layer

From data link layer

Network layer Network layer

TRANSPORT LAYER (PROCESS TO

PROCESS)

The transport layer is responsible for the delivery of a message from one process to another

Concerned:

Service-point addressing (Port address)

Segmentation and reassembly (Sequence number) 13

Segments

Data H4

From session layer

To network layer

Transport layer Data H4 Data H4

Segments

Data H4

From session layer

From network layer

Transport layer Data H4 Data H4

Connection control (Connectionless or connection oriented)

Flow control (end to end)

Error Control (Process to Process)

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SESSION LAYER (DIALOG INITIATION)

The session layer is responsible for dialog control and synchronization

Concerned:

Dialog Control (Half Duplex/Full duplex)

Synchronization (Synchronization points, process inline within same page)

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Data H5

From Presentation layer

To transport layer

Data Data

Syn Syn Syn

Data H5

To Presentation layer

From transport layer

Data Data

Syn Syn Syn

Session layer Session layer

PRESENTATION LAYER (DEPENDENCY)

The presentation layer is responsible for translation, compression and encryption

Concerned:

Translation (interoperability between different encoding system)

Encryption (Privacy schemes)

Compression (data compression)

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Data H6

From application layer

To session layer

Data H6

To application layer

From session layer

presentation layer presentation layer

APPLICATION LAYER (USER LEVEL

SERVICE)

The application layer is responsible for providing services to the user.

Concerned:

Network virtual terminal (Software) 17

Data H7

USER

(Human or Program)

To presentation layer

Data H7

Application layer

Application layer

Message Message

X.500 FTAM X.400 X.500 FTAM X.400

From presentation layer

USER

(Human or Program)

CONTINUE…

File transfer, access and management

Mail services

Directory services (access to distributed database sources for global information about various objects and services)

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2.2 TCP/ IP MODEL

The Internet Protocol Suite (commonly known as TCP/IP) is the set of communications protocols used for the Internet and other similar networks.

It is named from two of the most important protocols in it:

the Transmission Control Protocol (TCP) and

the Internet Protocol (IP), which were the first two networking protocols defined in this standard.

APPLICATION

This layer is comparable to the application,

presentation, and session layers of the OSI model

all combined into one.

It provides a way for applications to have access

to networked services.

This layer also contains the high level

protocols. The main issue with this layer is the

ability to use both TCP and UDP protocols.

For example TFTP uses UDP because usually on a

LAN the physical links are short enough to ensure

quick and reliable packet delivery without many

errors. SMTP instead uses TCP because of the error

checking capabilities.

Since we consider our email important information we

would like to ensure a safe delivery.

TFTP (Trivial File Transfer Protocol) -

Simplified version of the FTP protocol which has

no security features.

TRANSPORT

This layer acts as the delivery service used by the

application layer.

Again the two protocols used are TCP and UDP.

The choice is made based on the application's

transmission reliability requirements.

The transport layer also handles all error

detection and recovery.

It uses checksums, acknowledgements, and

timeouts to control transmissions and end to end

verification.

Unlike the OSI model, TCP/IP treats reliability

as an end-to-end problem

INTERNET LAYER(NETWORK

LAYER)

This layer is also known as Internet layer. The

main purpose of this layer is to organize or

handle the movement of data on network.

By movement of data, we generally mean

routing of data over the network. The main

protocol used at this layer is IP. While

ICMP(used by popular ‘ping’ command) and IGMP are also used at this layer.

HOST TO NETWORK INTERFACE

This layer is also known as network interface

layer

This layer normally consists of device drivers in

the OS and the network interface card attached

to the system.

Both the device drivers and the network

interface card take care of the communication

details with the media being used to transfer the

data over the network.

In most of the cases, this media is in the form of

cables.

Some of the famous protocols that are used at

this layer include ARP(Address resolution

protocol), PPP(Point to point protocol) etc.

2.3 CONNECTION ORIENTED V/S CONNECTIONLESS

APPROACH

Connection-oriented approach

Setup data transfer ahead of time (through

handshaking)

Internet’s connection-oriented service is TCP

(Transmission Control Protocol)

It provides reliable, in-order byte delivery

flow control

congestion control.

Applications using TCP: Email (SMTP), web

browsing (HTTP), and file transfer (FTP)

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CONTINUE Connectionless approach

Internet’s connectionless service is UDP (User Datagram Protocol) .

It provides unreliable data transfer.

no flow control

no congestion control

Applications using UDP: streaming media, video

conferencing, and IP telephony

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2.4 COMPARE OF OSI AND TCP/IP

OSI: Open Systems

Interconnection. It

was developed by ISO

as a first step toward

international

standardization of the

protocol used in

various layers. It

deals with connecting

open system..

TCP/IP: Transport

Control

Protocol/Internet

Protocol. TCP is used

in connection with IP

and operates at the

transport layer. IP is

the set of convention

used to pass packets

from one host to

another.

SIMILARITY

Both are based on the concept of a stack of

independent protocols.

The functionality of the layers is roughly similar.

DIFFERENCE

OSI makes the

distinction between

services, interfaces,

and protocol.

The OSI model was

devised before the

protocols were

invented. It can be

made to work in

diverse heterogeneous

networks.

TCP/IP does not

originally clearly

distinguish between

services, interface,

and protocol.

TCP/IP model was

just a description of

the existing protocols.

The model and the

protocol fit perfectly.

DIFFERENCE

(CONTINUE)

The OSI model

supports both

connectionless and

connection-oriented

communication in the

network layer, but

only connection-

oriented

communication in the

transport layer.

The TCP/IP model has

only one mode in the

network layer

(connectionless) but

supports both modes

in the transport layer,

giving the user choice.

DIFFERENCE

(CONTINUE)

OSI has seven layers TCP/IP has four layers

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Transport

Internet

Host-to-network

DIFFERENCE

(CONTINUE)

OSI emphasis on providing a reliable data transfer service, Each layer of the OSI model detects and handles errors, all data transmitted includes checksums. The transport layer checks source-destination reliability.

TCP/IP treats reliability as an end to end Problem. The transport layer handles all error detection and recovery, it was checksums, acknowledgments, and timeouts to control transmissions and provides end-to-end verification.

DIFFERENCE

(CONTINUE)

Host on OSI

implementations do

not handle network

operations.

TCP/IP hosts

participate in most

network protocols.